The Heart Failure Problem:
Congestive Heart Failure (CHF) is a form of heart disease still increasing in frequency. According to the American Heart Association, CHF is the “Disease of the Next Millennium”. The number of patients with CHF is expected to grow even more significantly as an increasing number of the “Baby Boomers” reach 50 years of age. CHF is a condition that occurs when the heart becomes damaged and reduces blood flow to the organs of the body. If blood flow decreases sufficiently, kidney function becomes impaired and results in fluid retention, abnormal hormone secretions and increased constriction of blood vessels. These results increase the workload of the heart and further decrease the capacity of the heart to pump blood through the kidney and circulatory system. This reduced capacity further reduces blood flow to the kidney, which in turn further reduces the capacity of the blood. It is believed that the progressively-decreasing perfusion of the kidney is the principal non-cardiac cause perpetuating the downward spiral of the “Vicious Cycle of CHF”. Moreover, the fluid overload and associated clinical symptoms resulting from these physiologic changes are predominant causes for excessive hospital admissions, terrible quality of life and overwhelming costs to the health care system due to CHF.
While many different diseases may initially damage the heart, once present, CHF is split into two types: Chronic CHF and Acute (or Decompensated-Chronic) CHF. Chronic Congestive Heart Failure is a longer term, slowly progressive, degenerative disease. Over years, chronic congestive heart failure leads to cardiac insufficiency. Chronic CHF is clinically categorized by the patient's ability to exercise or perform normal activities of daily living (such as defined by the New York Heart Association Functional Class). Chronic CHF patients are usually managed on an outpatient basis, typically with drugs.
Chronic CHF patients may experience an abrupt, severe deterioration in heart function, termed Acute Congestive Heart Failure, resulting in the inability of the heart to maintain sufficient blood flow and pressure to keep vital organs of the body alive. These acute CHF deteriorations can occur when extra stress (such as an infection or excessive fluid overload) significantly increases the workload on the heart in a stable chronic CHF patient. In contrast to the stepwise downward progression of chronic CHF, a patient suffering acute CHF may deteriorate from even the earliest stages of CHF to severe hemodynamic collapse. In addition, Acute CHF can occur within hours or days following an Acute Myocardial Infarction (AMI), which is a sudden, irreversible injury to the heart muscle, commonly referred to as a heart attack.
Normal Kidney Function:
The kidneys are a pair of organs that lie in the back of the abdomen on each side of the vertebral column. Kidneys play an important regulatory role in maintaining the homeostatic balance of the body. The kidneys function like a complex chemical plant. The kidneys eliminate foreign chemicals from the body, regulate inorganic substances and the extracellular fluid, and function as endocrine glands, secreting hormonal substances like renin and erythropoietin.
The main functions of the kidney are to maintain the water balance of the body and control metabolic homeostasis. Healthy kidneys regulate the amount of fluid in the body by making the urine more or less concentrated, thus either reabsorbing or excreting more fluid, respectively. In case of renal disease, some normal and important physiological functions become detrimental to the patient's health. This process is called overcompensation. In the case of Chronic Renal Failure (CRF) patients overcompensation often manifests in hypertension (pathologically high blood pressure) that is damaging to heart and blood vessels and can result in a stroke or death.
The functions of the kidney can be summarized under three broad categories: a) filtering blood and excreting waste products generated by the body's metabolism; b) regulating salt, water, electrolyte and acid-base balance; and c) secreting hormones to maintain vital organ blood flow. Without properly functioning kidneys, a patient will suffer water retention, reduced urine flow and an accumulation of wastes toxins in the blood and body.
The primary functional unit of the kidneys that is involved in urine formation is called the “nephron”. Each kidney consists of about one million nephrons. The nephron is made up of a glomerulus and its tubules, which can be separated into a number of sections: the proximal tubule, the medullary loop (loop of Henle), and the distal tubule. Each nephron is surrounded by different types of cells that have the ability to secrete several substances and hormones (such as renin and erythropoietin). Urine is formed as a result of a complex process starting with the filtration of plasma water from blood into the glomerulus. The walls of the glomerulus are freely permeable to water and small molecules but almost impermeable to proteins and large molecules. Thus, in a healthy kidney, the filtrate is virtually free of protein and has no cellular elements. The filtered fluid that eventually becomes urine flows through the tubules. The final chemical composition of the urine is determined by the secretion into and reabsorption of substances from the urine required to maintain homeostasis.
Receiving about 20% of cardiac output, the two kidneys filter about 125 ml of plasma water per minute. This is called the Glomerular Filtration Rate (GFR) and is the gold standard measurement of the kidney function. Since measurement of GFR is very cumbersome and expensive, clinically, the serum creatinine level or creatinine clearance are used as surrogates to measure kidney function. Filtration occurs because of a pressure gradient across the glomerular membrane. The pressure in the arteries of the kidney pushes plasma water into the glomerulus causing filtration. To keep the GFR relatively constant, pressure in the glomerulus is held constant by the constriction or dilatation of the afferent and efferent arterioles, the muscular walled vessels leading to and from each glomerulus.
Abnormal Kidney Function in CHF:
The kidneys maintain the water balance of the body and control metabolic homeostasis. The kidneys regulate the amount of fluid in the body by making the urine more or less concentrated, thus either reabsorbing or excreting more fluid, respectively. Without properly functioning kidneys, a patient will suffer water retention, reduced urine flow and an accumulation of wastes toxins in the blood and body. These conditions resulting from reduced renal function or renal failure (kidney failure) are believed to increase the workload of the heart. In a CHF patient, renal failure will cause the heart to further deteriorate as the water build-up and blood toxins accumulate due to the poorly functioning kidneys and in turn, cause the heart further harm.
In a CHF patient, for any of the known cause of heart dysfunction, the heart will progressively fail and blood flow and pressure will drop in the patients circulatory system. In the acute heart failure, the short-term compensations serve to maintain perfusion to critical organs, notably the brain and the heart that cannot survive prolonged reduction in blood flow. In chronic heart failure, these same responses that initially aided survival in acute heart failure can become deleterious.
A combination of complex mechanisms contribute to the deleterious fluid overload in CHF. As the heart fails and blood pressure drops, the kidneys cannot function owing to insufficient blood pressure for perfusion and become impaired. This impairment in renal function ultimately leads to a decrease in urine output. Without sufficient urine output, the body retains fluids and the resulting fluid overload causes peripheral edema (swelling of the legs), shortness of breath (from fluid in the lungs), and fluid in the abdomen, among other undesirable conditions in the patient.
In addition, the decrease in cardiac output leads to reduced renal blood flow, increased neurohormonal stimulus, and release of the hormone renin from the juxtaglomerular apparatus of the kidney. This results in avid retention of sodium and thus volume expansion. Increased rennin results in the formation of angiotensin, a potent vasoconstrictor.
Heart failure and the resulting reduction in blood pressure reduces the blood flow and perfusion pressure through organs in the body, other than the kidneys. As they suffer reduced blood pressure, these organs may become hypoxic causing the development of a metabolic acidosis which reduces the effectiveness of pharmacological therapy as well as increases the risk of sudden death.
This spiral of deterioration that physicians observe in heart failure patients is believed to be mediated, in large part, by activation of a subtle interaction between heart function and kidney function, known as the renin-angiotensin system. Disturbances in the heart's pumping function results in decreased cardiac output and diminished blood flow. The kidneys respond to the diminished blood flow as though the total blood volume was decreased, when in fact the measured volume is normal or even increased. This leads to fluid retention by the kidneys and formation of edema causing fluid overload and increased stress on the heart.
Systemically, CHF is associated with an abnormally elevated peripheral vascular resistance and is dominated by alterations of the circulation resulting from an intense disturbance of sympathetic nervous system function. Increased activity of the sympathetic nervous system promotes a downward vicious cycle of increased arterial vasoconstriction (increased resistance of vessels to blood flow) followed by a further reduction of cardiac output, causing even more diminished blood flow to the vital organs.
In CHF via the previously explained mechanism of vasoconstriction, the heart and circulatory system dramatically reduces blood flow to kidneys. During CHF, the kidneys receive a command from higher neural centers via neural pathways and hormonal messengers to retain fluid and sodium in the body. In response, to stress on the heart, the neural centers command the kidneys to reduce their filtering functions. While in the short term, these commands can be beneficial, if these commands continue over hours and days they can jeopardize the person's life or make the person dependent on artificial kidney for life by causing the kidneys to cease functioning.
When the kidneys do not fully filter the blood, a huge amount of fluid is retained in the body resulting in bloating (fluid in tissues), and increases the workload of the heart. Fluid can penetrate into the lungs and the patient becomes short of breath. This odd and self-destructive phenomenon is most likely explained by the effects of normal compensatory mechanisms of the body that improperly perceive the chronically low blood pressure of CHF as a sign of temporary disturbance such as bleeding.
In an acute situation, the organism tries to protect its most vital organs, the brain and the heart, from the hazards of oxygen deprivation. Commands are issued via neural and hormonal pathways and messengers. These commands are directed toward the goal of maintaining blood pressure to the brain and heart, which are treated by the body as the most vital organs. The brain and heart cannot sustain low perfusion for any substantial period of time. A stroke or a cardiac arrest will result if the blood pressure to these organs is reduced to unacceptable levels. Other organs, such as kidneys, can withstand somewhat longer periods of ischemia without suffering long-term damage. Accordingly, the body sacrifices blood supply to these other organs in favor of the brain and the heart.
The hemodynamic impairment resulting from CHF activates several neurohormonal systems, such as the renin-angiotensin and aldosterone system, sympatho-adrenal system and vasopressin release. As the kidneys suffer from increased renal vasoconstriction, the filtering rate (GFR) of the blood drops and the sodium load in the circulatory system increases. Simultaneously, more renin is liberated from the juxtaglomerular of the kidney. The combined effects of reduced kidney functioning include reduced glomerular sodium load, an aldosterone-mediated increase in tubular reabsorption of sodium, and retention in the body of sodium and water. These effects lead to several signs and symptoms of the CHF condition, including an enlarged heart, increased systolic wall stress, an increased myocardial oxygen demand, and the formation of edema on the basis of fluid and sodium retention in the kidney. Accordingly, sustained reduction in renal blood flow and vasoconstriction is directly responsible for causing the fluid retention associated with CHF.
In view of the physiologic mechanisms described above it is positively established that the abnormal activity of the kidney is a principal non-cardiac cause of a progressive condition in a patient suffering from CHF.
Growing population of late stage CHF patients is an increasing concern for the society. The disease is progressive, and as of now, not curable. The limitations of drug therapy and its inability to reverse or even arrest the deterioration of CHF patients are clear. Surgical therapies are effective in some cases, but limited to the end-stage patient population because of the associated risk and cost. There is clearly a need for a new treatment that will overcome limitations of drug therapy but will be less invasive and costly than heart transplantation.
Similar condition existed several decades ago in the area of cardiac arrhythmias. Limitations of anti-arrhythmic drugs were overcome by the invention of heart pacemakers. Widespread use of implantable electric pacemakers resulted in prolonged productive life for millions of cardiac patients. So far, all medical devices proposed for the treatment of CHF are cardio-centric i.e., focus on the improvement of the heart function. The dramatic role played by kidneys in the deterioration of CHF patients has been overlooked by the medical device industry.
Neural Control of Kidneys:
The autonomic nervous system is recognized as an important pathway for control signals that are responsible for the regulation of body functions critical for maintaining vascular fluid balance and blood pressure. The autonomic nervous system conducts information in the form of signals from the body's biologic sensors such as baroreceptors (responding to pressure and volume of blood) and chemoreceptors (responding to chemical composition of blood) to the central nervous system via its sensory fibers. It also conducts command signals from the central nervous system that control the various innervated components of the vascular system via its motor fibers.
Experience with human kidney transplantation provided early evidence of the role of the nervous system in the kidney function. It was noted that after the transplant, when all the kidney nerves are totally severed, the kidney increased the excretion of water and sodium. This phenomenon was also observed in animals when the renal nerves were cut or chemically destroyed. The phenomenon was called “denervation diuresis” since the denervation acted on a kidney similar to a diuretic medication. Later the “denervation diuresis” was found to be associated with the vasodilatation the renal arterial system that led to the increase of the blood flow through the kidney. This observation was confirmed by the observation in animals that reducing blood pressure supplying the kidney could reverse the “denervation diuresis”.
It was also observed that after several months passed after the transplant surgery in successful cases, the “denervation diuresis” in transplant recipients stopped and the kidney function returned to normal. Originally it was believed that the “renal diuresis” is a transient phenomenon and that the nerves conducting signals from the central nervous system to the kidney are not essential for the kidney function. Later, new discoveries led to the different explanation. It is believed now that the renal nerves have a profound ability to regenerate and the reversal of the “denervation diuresis” shall be attributed to the growth of the new nerve fibers supplying kidneys with the necessary stimuli.
Another body of research that is of particular importance for this application was conducted in the period of 1964-1969 and focused on the role of the neural control of secretion of the hormone renin by the kidney. As was discussed previously, renin is a hormone responsible for the “vicious cycle” of vasoconstriction and water and sodium retention in heart failure patients. It was demonstrated that increase (renal nerve stimulation) or decrease (renal nerve denervation) in renal sympathetic nerve activity produced parallel increases and decreases in the renin secretion rate by the kidney, respectively.
In summary, it is known from clinical experience and the large body of animal research that the stimulation of the renal nerve leads to the vasoconstriction of blood vessels supplying the kidney, decreased renal blood flow, decreased removal of water and sodium from the body and increased renin secretion. These observations closely resemble the physiologic landscape of the deleterious effects of the chronic congestive heart failure. It is also known that the reduction of the sympathetic renal nerve activity, achieved by denervation, can reverse these processes.
It was established in animal models that the heart failure condition results in the abnormally high sympathetic stimulation of the kidney. This phenomenon was traced back to the sensory nerves conducting signals from baroreceptors to the central nervous system. Baroreceptors are the biologic sensors sensitive to blood pressure. They are present in the different locations of the vascular system. Powerful relationship exists between the baroreceptors in the carotid arteries (supplying brain with arterial blood) and the sympathetic nervous stimulus to the kidneys. When the arterial blood pressure was suddenly reduced in experimental animals with heart failure, the sympathetic tone increased. Nevertheless the normal baroreflex alone, cannot be responsible for the elevated renal nerve activity in chronic CHF patients. If exposed to the reduced level of arterial pressure for a prolonged time baroreceptors normally “reset” i.e. return to the baseline level of activity until a new disturbance is introduced. Therefore, in chronic CHF patients the components of the autonomic nervous system responsible for the control of blood pressure and the neural control of the kidney function become abnormal. The exact mechanisms that cause this abnormality are not fully understood but, its effects on the overall condition of the CHF patients are profoundly negative.
End Stage Renal Disease Problem:
There is a dramatic increase in patients with end-stage renal disease (ESRD) due to diabetic nephropathy, chronic glomerulonephritis and uncontrolled hypertension. In the US alone, 372,000 patients required dialysis in the year 2000. There were 90,000 new cases of ESRD in 1999 with the number of patients on dialysis is expected to rise to 650,000 by the year 2010. The trends in Europe and Japan are forecasted to follow a similar path. Mortality in patients with ESRD remains 10-20 times higher than that in the general population. Annual Medicare patient costs $52,868 for dialysis and $18,496 for transplantation. The total cost for Medicare patients with ESRD in 1998 was $12.04 billion.
The primary cause of these problems is the slow relentless progression of Chronic Renal Failure (CRF) to ESRD. CRF represents a critical period in the evolution of ESRD. The signs and symptoms of CRF are initially minor, but over the course of 2-5 years, become progressive and irreversible. Until the 1980's, there were no therapies that could significantly slow the progression of CRF to ESRD. While some progress has been made in combating the progression to and complications of ESRD in last two decades, the clinical benefits of existing interventions remain limited with no new drug or device therapies on the horizon.
Progression of Chronic Renal Failure:
It has been known for several decades that renal diseases of diverse etiology (hypotension, infection, trauma, autoimmune disease, etc.) can lead to the syndrome of CRF characterized by systemic hypertension, proteinuria (excess protein filtered from the blood into the urine) and a progressive decline in GFR ultimately resulting in ESRD. These observations suggested that CRF progresses via a common pathway of mechanisms, and that therapeutic interventions inhibiting this common pathway may be successful in slowing the rate of progression of CRF irrespective of the initiating cause.
To start the vicious cycle of CRF, an initial insult to the kidney causes loss of some nephrons. To maintain normal GFR, there is an activation of compensatory renal and systemic mechanisms resulting in a state of hyperfiltration in the remaining nephrons. Eventually, however, the increasing numbers of nephrons “overworked” and damaged by hyperfiltration are lost. At some point, a sufficient number of nephrons are lost so that normal GFR can no longer be maintained. These pathologic changes of CRF produce worsening systemic hypertension, thus high glomerular pressure and increased hyperfiltration. Increased glomerular hyperfiltration and permeability in CRF pushes an increased amount of protein from the blood, across the glomerulus and into the renal tubules. This protein is directly toxic to the tubules and leads to further loss of nephrons, increasing the rate of progression of CRF. This vicious cycle of CRF continues as the GFR drops, with loss of additional nephrons leading to further hyperfiltration and eventually to ESRD requiring dialysis. Clinically, hypertension and excess protein filtration have been shown to be two major determining factors in the rate of progression of CRF to ESRD.
Though previously clinically known, it was not until the 1980s that the physiologic link between hypertension, proteinuria, nephron loss and CRF was identified. In 1990s the role of sympathetic nervous system activity was elucidated. Afferent signals arising from the damaged kidneys due to the activation of mechanoreceptors and chemoreceptors stimulate areas of the brain responsible for blood pressure control. In response brain increases sympathetic stimulation on the systemic level resulting in the increased blood pressure primarily through vasoconstriction of blood vessels.
When elevated sympathetic stimulation reaches the kidney via the efferent sympathetic nerve fibers, it produces major deleterious effects in two forms:
A. Kidney is damaged by direct renal toxicity from the release of sympathetic neurotransmitters (such as norepinephrine) in the kidney independent of the hypertension.
B. Secretion of renin that activates Angiotensin II is increased leading to the increased systemic vasoconstriction and exacerbated hypertension.
Over time damage to the kidney leads to further increase of afferent sympathetic signals from the kidney to the brain. Elevated Angiotensin II further facilitates internal renal release of neurotransmitters. The feedback loop is therefore closed accelerating the deterioration of the kidney.